This invention relates to a temperature control device in use for the fixing device of an image-forming apparatus, which can detect a disconnection occurring in a high-voltage circuit of the commercial power supply, in a low-voltage temperature control circuit.
To perform temperature control for the fixing device of an image-forming apparatus such as an electronic photocopying machine, it is necessary for the fixing temperature to be kept constant. Accordingly, various temperature control circuits have been developed for this purpose. In one such example of a temperature control circuit, the temperature of the peripheral surface of a heating roller is detected by means of a thermo-sensitive element such as a thermistor. Based on the result, power to the heating element, such as a heater, is turned on and off to maintain the optimum temperature level for fixing. FIG. 1 shows a typical conventional temperature control circuit for a fixing device. This control circuit is composed of a high-voltage circuit having a commercial power supply circuit with a heating element, and a low-voltage control circuit having a thermo-sensitive element. In this temperature control circuit, the circuit for detecting the disconnection of the heating element is usually contained in the high-voltage circuit.
The circuit illustrated in FIG. 1 is constructed having a bridge circuit which is composed of thermistor TH, resistors R.sub.1, R.sub.2, and R.sub.3, a temperature control section made up of comparator Q.sub.1, trigger circuit 1, transistor Q.sub.2, and resistors R.sub.4 to R.sub.6, and a heater-disconnection detecting section having a triac 2, a heater H, alternating-current power supply 3, and current-detecting circuit 4.
In this circuit, comparator Q.sub.1 compares the voltage V.sub.N obtained, by dividing the power voltage Vcc by resistor R.sub.3 and thermistor T.sub.H, and the reference voltage V.sub.I obtained, by dividing it by resistors R.sub.1 and R.sub.2. The compared voltage turns triac 2 on and off via transistor Q.sub.2 and trigger circuit 1. In this way, the temperature control circuit controls the feed of AC current to the heater. During this operation, current-detecting circuit 4 monitors the output of comparator Q.sub.1. When the output of comparator Q.sub.1 is a high signal (which is output if the current, which should be supplied by the closed-loop circuit, is not supplied from AC power supply 3), current-detecting circuit 4 outputs a disconnection signal A, to indicate disconnection of the heater. The conventional temperature control device, as described above, is capable of detecting the disconnection of heater H. However, current-detecting circuit 4 must be designed so as to withstand high voltage, since it is coupled with AC power supply 3, or a commercial power supply. Such a circuit is inevitably costly to manufacture. Furthermore, the conventional temperature control circuit has to be designed so as to satisfy the power supply specifications of each country of export. This means that standardization of parts cannot be attained, so that the inefficient production of small quantities of many different types of parts is therefore unavoidable.